Stress State on the North Anatolian Fault Thirteen Years After the 1999 M7.4 Izmit Earthquake

The stress state around the western section of the North Anatolian Fault (NAF) that broke in 1999 during the Izmit earthquake was inferred from seismicity just before and after the event (e.g. Pnar et al., 2010; Ickrath et al., 2015). These studies offered a detailed description of how the stress field changed, in time and space, following the Izmit earthquake. All studies described the weakening of the fault (one of the principal stresses is perpendicular to the fault) and the stress rotation induced by the rupture. We used our catalog of microseismicity based on the data collected by the dense array DANA in 2012-2013 to compute (composite) focal mechanisms. We used our iterative linear stress inversion method to infer the stress state along different segments of the NAF from the focal mechanisms. Our results show that the stress state, though different from the early postseismic stress state, had not returned to its preseismic state at the time the data were collected. We discuss the implications of our observations on the time evolution of the frictional properties of the fault, and other postseismic phenomena at play.